Field of the Invention
The invention relates generally to the art of axle/suspension systems for heavy-duty vehicles. More particularly, the invention relates to air-ride axle/suspension systems for heavy-duty vehicles which utilize a damping air spring to cushion the ride of the vehicle. More specifically, the invention is directed to the combination of a damping air spring utilized in conjunction with a shock absorber for heavy-duty vehicle air-ride axle/suspension systems, in which the damping air spring is optimized to aid in providing damping characteristics to the axle/suspension system at a selected frequency range, and the shock absorber is optimized to aid in providing damping to the axle/suspension system at a selected frequency range generally different from the frequency range damped by the damping air spring. The combination of the damping air spring and shock absorber working together supplement one another to provide optimized damping across the entire range of critical frequencies encountered by the axle/suspension system during operation, thereby increasing the soft ride characteristics of the axle/suspension system and extending the life of the components of the axle/suspension system, tires and other vehicle components, as well as potentially reducing weight of the axle/suspension system.
Background Art
The use of one or more air-ride trailing and leading arm rigid beam-type axle/suspension systems has been very popular in the heavy-duty truck, bus and tractor-trailer industry for many years. Although such axle/suspension systems can be found in widely varying structural forms, in general their structure is similar in that each system typically includes a pair of suspension assemblies. In some heavy-duty vehicles, the suspension assemblies are connected directly to the primary frame of the vehicle. In other heavy-duty vehicles, the primary frame of the vehicle supports a subframe, and the suspension assemblies connect directly to the subframe. For those heavy-duty vehicles that support a subframe, the subframe can be non-movable or movable, the latter being commonly referred to as a slider box, slider subframe, slider undercarriage, or secondary slider frame. For the purpose of convenience and clarity, reference herein will be made to main members, with the understanding that such reference is by way of example, and that the present invention applies to heavy-duty vehicle axle/suspension systems suspended from main members of: primary frames, movable subframes and non-movable subframes.
Specifically, each suspension assembly of an axle/suspension system includes a longitudinally extending elongated beam. Each beam typically is located adjacent to and below a respective one of a pair of spaced-apart longitudinally extending main members and one or more cross members which form the frame of the vehicle. More specifically, each beam is pivotally connected at one of its ends to a hanger, which in turn is attached to and depends from a respective one of the main members of the vehicle. An axle extends transversely between and typically is connected by some means to the beams of the pair of suspension assemblies at a selected location from about the mid-point of each beam to the end of the beam opposite from its pivotal connection end. The beam end opposite the pivotal connection end also is connected to an air spring, or its equivalent, which in turn is connected to a respective one of the main members. The air springs cushion the ride of the vehicle during operation, and in some cases, provide damping. A height control valve is mounted on the main member or other support structure and is operatively connected to the beam and to the air spring in order to maintain the ride height of the vehicle. A brake system and one or more shock absorbers also are mounted on the axle/suspension system. The shock absorbers provide damping to the axle/suspension system of the vehicle during operation. The beam may extend rearwardly or frontwardly from the pivotal connection relative to the front of the vehicle, thus defining what are typically referred to as trailing arm or leading arm axle/suspension systems, respectively. However, for purposes of the description contained herein, it is understood that the term “trailing arm” will encompass beams which extend either rearwardly or frontwardly with respect to the front end of the vehicle.
The axle/suspension systems of the heavy-duty vehicle act to cushion the ride, dampen vibrations and stabilize the vehicle. More particularly, as the vehicle is traveling over the road, its wheels encounter road conditions that impart various forces, loads, and/or stresses, collectively referred to herein as forces, to the respective axle on which the wheels are mounted, and in turn, to the suspension assemblies that are connected to and support the axle. In order to minimize the detrimental effect of these forces on the vehicle as it is operating, the axle/suspension system is designed to react and/or absorb at least some of the forces.
These forces include vertical forces caused by vertical movement of the wheels as they encounter certain road conditions, fore-aft forces caused by acceleration and deceleration of the vehicle, and side-load and torsional forces associated with transverse vehicle movement, such as turning of the vehicle and lane-change maneuvers. In order to address such disparate forces, axle/suspension systems have differing structural requirements. More particularly, it is desirable for an axle/suspension system to be fairly stiff in order to minimize the amount of sway experienced by the vehicle and thus provide what is known in the art as roll stability. However, it is also desirable for an axle/suspension system to be relatively flexible to assist in cushioning the vehicle from vertical impacts, and to provide compliance so that the components of the axle/suspension system resist failure, thereby increasing durability of the axle/suspension system. It is also desirable to dampen the vibrations or oscillations that result from such forces.
A key component of the axle/suspension system that cushions the ride of the vehicle from vertical impacts is the air spring, while a shock absorber typically provides damping characteristics to the axle/suspension system, although air springs with damping features have also been utilized.
The typical air spring without damping features of the type utilized in heavy-duty air-ride axle/suspension systems includes three main components: a flexible bellows, a piston and a bellows top plate. The bellows is typically formed from rubber or other flexible material, and is operatively mounted on top of the piston. The piston is typically formed from steel, aluminum, fiber reinforced plastic or other rigid material and is mounted on the rear end of the top plate of the beam of the suspension assembly by fasteners, which are generally well known in the art. The volume of pressurized air, or “air volume”, that is contained within the air spring is a major factor in determining the spring rate of the air spring. More specifically, this air volume is contained within the bellows and, in some cases, the piston of the air spring. The larger the air volume of the air spring, the lower the spring rate of the air spring. A lower spring rate is generally more desirable in the heavy-duty vehicle industry because it provides a softer ride to the vehicle during operation. Typically, the piston either contains a hollow cavity, which is in communication with the bellows and which adds to the air volume of the air spring by allowing unrestricted communication of air between the piston and the bellows volumes, or the piston has a generally hollow cylindrical-shape and does not communicate with the bellows volume, whereby the piston does not contribute to the air volume of the air spring. The air volume of the air spring is in fluid communication with an air source, such as an air supply tank, and also is in fluid communication with the height control valve of the vehicle. The height control valve, by directing air flow into and out of the air spring of the axle/suspension system, helps maintain the desired ride height of the vehicle.
Prior art air springs such as the one described above, while providing cushioning to the vehicle, cargo and occupant(s) during operation of the vehicle, provide little, if any, damping to the axle/suspension system. Such damping is instead typically provided by a pair of hydraulic shock absorbers, although a single shock absorber has also been utilized and is generally well known in the art. The shock absorber typically includes a cylinder that is filled with fluid. A plunger with a diaphragm mounted on its end is disposed longitudinally within the fluid filled cylinder so that the plunger and diaphragm can move within the fluid filled cylinder. The diaphragm typically includes a number of openings and also includes a blow off valve that is mounted on the diaphragm. The blow off valve includes larger openings that allow a two-stage damping curve that is generally well known in the art. The shock absorber cylinder is mounted to the beam of a respective one of the suspension assemblies and the plunger is mounted to a respective one of the main members of the vehicle. As the beam is rotated upwardly toward the main member during operation of the vehicle, the plunger and diaphragm are moved downwardly through the fluid filled cylinder. As the beam is rotated downwardly away from the main member during operation of the vehicle, the plunger and diaphragm are moved upwardly through the fluid filled cylinder. The movement of the plunger and diaphragm through the fluid filled cylinder results in viscous damping of the axle/suspension system.
For trailers of heavy-duty vehicles, the frequencies where optimal damping of the axle/suspension system(s) is critical are from about 1.8 Hz, body bounce mode, to about 13 Hz, wheel hop mode. At these natural frequencies, the axle/suspension system is predisposed to move, so road inputs at these frequencies can result in a build-up of movement in the axle/suspension system that can potentially adversely affect the performance of the axle/suspension system.
Prior art shock absorbers have a continuously increasing damping curve at higher frequencies. This means that as the frequency of the inputs on the axle/suspension system increase, the damping provided by the shock absorber to the axle/suspension system is increased. This increased damping at higher input frequencies causes increased transmissibility of the forces acting on the axle/suspension system through the shock absorbers, which in turn can reduce the soft ride characteristics of the axle/suspension system at higher frequencies and can also lead to premature wear of the components of the axle/suspension system, tires and other vehicle components. The continuously increasing damping curve of the prior art shock absorbers can also potentially cause “misting” of the shock absorber. More specifically, misting occurs when fluid contained in the shock absorber is forced out around the plunger of the shock absorber during operation of the shock absorber. This occurs when high energy road inputs are imparted to the axle/suspension system through the wheels of the vehicle during operation of the vehicle. These high energy inputs on the axle/suspension system cause a larger damping force in the prior art shock absorber because of the continuously increasing damping curve of shock absorbers. This increased damping force in turn causes the shock absorber to generate very high internal pressures, which can cause the shock absorber to mist. Although misting of the shock absorber does not typically compromise the ability of the shock absorber to provide damping, misting can adversely affect components surrounding the shock absorber by contaminating them with fluid from the shock absorber and also may result in the belief that the shocks or other components have failed, when they have not, resulting in premature replacement of the shocks or other components which adds additional costs. Therefore, reducing the amount of misting of the shock absorber is preferred in order to minimize the possibility of contamination of surrounding components and premature replacement of the shocks or other vehicle components. In addition, the increased damping force of the prior art shock absorber at higher frequencies can cause increased stress to the components of the axle/suspension system that can in turn potentially increase wear and reduce the life of the components of the axle/suspension system, tires and other vehicle components.
The amount of cargo that a vehicle may carry is governed by local, state, and/or national road and bridge laws. The basic principle behind most road and bridge laws is to limit the maximum load that a vehicle may carry, as well as to limit the maximum load that can be supported by individual axles. Because standard prior art shock absorbers are relatively heavy, these components add undesirable weight to the axle/suspension system and therefore reduce the amount of cargo that can be carried by the heavy-duty vehicle. Depending on the shock absorbers employed, they also add varying degrees of complexity to the axle/suspension system, which is also undesirable.
Air springs with damping features, such as the one described in U.S. Pat. No. 8,540,222 owned by the assignee of the present application, are also known. The air spring with damping features shown and described in the '222 patent can be incorporated into axle/suspension systems, such as the one described above, and includes a bellows and a piston. The top end of the bellows is sealingly engaged with a bellows top plate. An air spring mounting plate is mounted on the top portion of the top plate by fasteners, which are also used to mount the top portion of the air spring to a respective one of the main members of the vehicle. The piston is generally cylindrical-shaped and includes a continuous generally stepped sidewall attached to a generally flat bottom plate and integrally formed with a top plate. The piston bottom plate is formed with a central opening. A fastener is disposed through the opening in order to attach the piston to the beam top plate at the rear end of the beam.
The top plate, sidewall and bottom plate of the piston define a piston chamber having an interior volume. The piston top plate is formed with a circular upwardly extending protrusion having a lip around its circumference. The lip cooperates with the lowermost end of the bellows to form an airtight seal between the bellows and the lip. Alternate means of attachment are also known and are commonly used in the art. The bellows, top plate and piston top plate define a bellows chamber having an interior volume. The piston top plate is formed with a pair of openings, which allow the volume of the piston chamber and the volume of the bellows chamber to communicate with one another. The piston chamber volume, the bellows chamber volume and the cross-sectional area of the openings formed in the piston top plate between the piston chamber and the bellows chamber provide damping characteristics to the air spring during operation of the vehicle.
Other prior art air springs have attempted to provide damping characteristics to the air spring by placing valves between the bellows and piston chambers of the air spring. Still other prior art air springs have attempted to provide damping characteristics to the air spring by forming an opening between the bellows and piston chambers of the air spring which is partially covered by rubber flaps mounted adjacent to the opening.
These prior art air springs with damping features may potentially provide less than optimal damping at higher frequencies above about 5 Hz, which can in turn potentially cause reduced life of the components of the axle/suspension system, including potentially increased tire wear and payload damage.
The combination damping air spring and shock absorber of the present invention overcomes the problems associated with prior art damping air springs and shock absorbers utilized with non-damping air springs, by providing an optimized damping air spring that is utilized in combination with an optimized shock absorber resulting in improved damping characteristics for the axle/suspension system across the entire spectrum of critical input frequencies. The combination damping air spring and shock absorber for heavy-duty vehicles allows tuning of certain structural components of the shock absorber to provide optimal damping at higher frequencies, resulting in improved damping to the axle/suspension system while reducing transmissibility of forces and misting that are common in prior art standard shock absorbers, saving weight and cost, and enabling the heavy-duty vehicle to haul more cargo. Moreover, reducing transmissibility of road inputs into the components of the axle/suspension system during operation of the vehicle increases the durability of the axle/suspension system and the components of the axle/suspension system, while maintaining the soft ride characteristics of the axle/suspension system at higher critical input frequencies. In addition, the damping air spring provides optimal damping at lower frequencies below about 5 Hz, which in turn increases the life of components of the axle/suspension system, tires and other vehicle components.